Minimum Design Metal Temperature Presentation

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    Minimum Design Metal

    Temperature (MDMT)

    Presentation: Thai TruongApr 17, 2014

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    Brittle Fracture Phenomena

    Definition of Brittle Fracture

    Brittle fracture is a sudden, often catastrophic failure which is inherent tobrittle materials. It involves little or no deformation, and has been

    experienced in pressure vessels, tanks, and pipes.

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    Brittle Fracture Phenomena

    In some reactions and processes, loss of process control can result in asignificant change in temperature and/or pressure. The result can exceedthe intended limits of the materials selected. Thus, where cryogenic fluidsare being processed, a reduction in pressure could lower the temperatureof the fluids to a level below the minimum allowable design temperature ofthe equipment, with the attendant risk of a low-temperature brittle fracture.

    An example of this phenomena is when high levels of pressurized gasesare released into a piping system. The quick depressurization creates thepossibility for auto-regeneration of the piping materials that may cause

    brittle fracture.

    Below are typical depressurization times for high pressure systems:

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    Brittle Fracture Phenomena

    At normal or high temperatures, a warning is normally given by plasticdeformation as signs of potential vessel failure:

    - Bulging- Stretching

    - Yielding- Cracking- Leaking

    At low temperature condition, no such warnings of plastic deformation

    are given. An abrupt fracture at low temperature condition can cause acatastrophic event due to fragmentation of the structure and fastrelease of energy.

    It is a major concern in failure of pressure vessels, heat exchangers,and piping.

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    Brittle Fracture Phenomena

    As the metal temperature decreases, many metals loose their ductility andtoughness. They become susceptible to brittle fracture.

    Brittleness indicates that the material is prone to failure without

    deformation. Examples of brittle materials:- Chalk - Brick- Glass - Hardened steels

    Brittle materials are prone to fracture when they are stressed in the vicinityof a notch or stress concentration. Brittle fractures are infrequent, mostoccur during hydrotest rather than in operation.

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    Brittle Fracture Phenomena

    Toughness is the opposite of Brittleness, is the materials ability to resistbrittle fracture.

    Toughness depends on:

    - material strength- thickness

    - temperature.

    To resist brittle fracture, higher strength materials and thick materialsrequire greater toughness than low strength and thin materials.

    Steel lose toughness as temperature decreases.

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    Brittle Fracture Phenomena

    Four main factors, in combination, can cause brittle fracture of steelvessels:

    - low temperature

    - loading- susceptible steel

    - crack / stress risers

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    Brittle Fracture Phenomena

    Low Temperature FactorA metal depending on its toughness property has a transition temperaturerange within which it is in a semi-brittle condition ductile to brittle transition.Although the transition occurs over a temperature range, a point within this

    range is selected as the transition temperature to delineate the boundaries ofductile and brittle zone. The transition temperature is usually taken as thepoint where 50% of the fracture is brittle.

    One of the ways to determine this temperature is by performing a series of

    Charpy impact tests on materials.

    Above the transition range, brittle fracture will not happen, even a notch exists.

    Below the transition range, brittle fracture can happen, even no notch exists.

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    Brittle Fracture Phenomena

    The absorbed energy (Joule) is plotted against testing temperature, givinga ductile to brittle transition temperature curve. Test temperature rangeshall be wide enough to establish the upper and lower shelf energies, withsufficient testing at intermediate temperatures to permit plotting areasonable smooth curve.

    The curve represents a change in fracture behavior from ductile at hightemperature to brittle at lower temperature.

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    Brittle Fracture Phenomena

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    Brittle Fracture Phenomena

    Loading FactorDynamic loading associated with mechanical/thermal , impact loading,cyclic loading, or rapid decreases in equipment temperatures is acontributing factor.

    Susceptible Steel Factor Steel composition. Steel with lower carbon content (C %) have higher toughness.

    Phosphorus (P %) has a strong effect in raising the transition temperature and improvesweldability. Steel transition temperature is a function of carbon content plus 20 times thepercent of P.

    Steel structure. Grain size has a strong effect on transition temperature. Decreasing thegrain diameter from ASTM grain size 5 to fine grain size 10 can change the 10 ft/lb CharpyV-notch transition temperature from about 39C to -33C (70F to -60F).

    Hydrogen cracks. When hydrogen atoms diffuse into the metal during materialmanufacturing operations such as forming, forging and welding or when hydrogen isintroduced to the metal through a galvanic or hydrogen sulfide (H2S) corrosion process,

    the metal is prone to hydrogen cracks.

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    Brittle Fracture Phenomena

    Crack/Stress risers Steel vessels with thicker walls have a greater probability potential for

    brittle fracture due to the larger thermal gradient across the wallthickness. Thicker metal walls can result in differential expansion of

    material across the wall thickness and could possibly lead to a crackoccurrence and eventually brittle fracture.

    Stress raisers such as sharp or abrupt transitions or changes ofsections, corners or notches (as may be found in weld defects) as aresult of design or fabrication processes are all stress risers, which can

    cause stress intensification. The weak points are prone to brittle fracturewhen other susceptible conditions exist.

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    ASME CODE - MDMT

    MDMT of Vessels.The Minimum Design Metal Temperature (MDMT) of a vessel is theminimum metal temperature in which the vessel can sustain its full designpressure without having to be impact tested. When the vessel operates at

    pressures less than its full design pressure, concessions on MDMT areallowed based on ASME Section VIII.

    Minimum Allowable Temperature (MAT), as defined in API 579, is thelowest (coldest) permissible metal temperature for a given material andthickness based on its resistance to brittle fracture. It may be a singletemperature or an envelope of allowable operating temperatures as afunction of pressure. The MAT is derived from mechanical designinformation and material specification. MAT at design pressure is MDMT.

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    ASME CODE - MDMT

    Lowest Metal Temperature (LMT) is the lowest metal temperature due tothe operating condition and minimum ambient temperature. LMT may be asingle temperature at an operating pressure or an envelope oftemperatures and coincident pressures. LMT is derived from the calculatedinner wall temperature due to the contained process fluid temperature andalso the minimum ambient temperature.

    Determine the MDMT API 510 Welded Storage Tank

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    ASME CODE - MDMT

    Determine the MDMT ASMETo establish a minimum design metal temperature for new equipment,startup temperature and reasonably expected abnormal operatingtemperatures, including autorefrigeration should be considered, as well as

    normal operation.

    The best available local weather data should be used to establish startuptemperatures if the equipment is not normally preheated. If localtemperature data are not available, the lowest 1-day mean temperatureshown in Figure 4-2 of API-650 can be used.

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    ASME CODE - MDMT

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    ASME CODE - MDMT

    Fracture initiation in steels was foundto be difficult above a transitiontemperature corresponding to aCVN impact energy of 10 ft-lb.

    Crack propagation was found to be

    difficult above a temperaturecorresponding to 15 to 25 ft-lb.

    From these findings, a 15 ft-lb CVNrequirement at the minimum loading

    temperatures became a widely usedfracture criterion.

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    ASME CODE - MDMT

    Materials Selection Requirement.

    One of two methods is used to assuresteels are used above their transitiontemperature: Impact test exemption curves Charpy V-Notch impact testing

    A steel has adequate toughness if theapplication point is above